US2298466A - Electro-optical system and apparatus - Google Patents

Description

0ct. 13, 1942. A. G. coLEY ELECTRO-much ss'rnu .mn APPARATUS Fnd .my 1a, 1940: 2 sheets-Smm 1 l I'NvEN-ron 13, 1942 o A. s. com lsrV 2,298,466

ELECTRO-Omron, SYSTEM AND APPARATUS Filed July la, i940 2 snets-sheet 2 JJ l .7 v A 4f A 44V A yr f ffg,

l 'r l s E la 4 INVENTOR" ATTO RN Patented Oct. 13, 1942 UNITED STATES PATENT lOFI-lcs Austin G. Cooley, New York, N. Y., assignor, by

mesne assignments, to Times Telephoto Equipment Inc., New York New York, N. Y., a corporation of Application July 18, 1940, Serial No. 346,113

' 10 Claims. (Cl. 179-1715) This invention relates to electro-optical systems and more particularly to methods of converung iight signes into a momsted carrier current signal.

A principal object is to provide a -novel form of modulating arrangement suitable for use in tele-facsimile systems, television systems and electro-optical systems generally.

Another object is to provide a photo-electric cell translating system which has a maximum of sensitivity in responding to light signal variations.

Another object is to providel a novel method of employing a photo-electric cell or similar light sensitive device whereby the employment of the usual cell polarizing batteries or the like is obviated.

photo-electric cell bridge modulating arrangement.

A further feature relates to a novel photo- A further feature relates to anovel form of Heretofore, numerous 'circuit arrangements have been proposed for using a light sensitive device such as a photo cell as a light signal translator. In most of these prior arrangements, the

inherent interelectrode electrostatic capacity or the inherent electrostatic capacity of the electrodes with respect to-ground has been a drawb ack to the attainment of maximum sensitivity. It has therefore been proposed to overcome the undesirable eects of this inherent electrostatic capacity by connecting the photo-electric cell in Wheatstone bridge balancing network. An arrangement of this kind is described in Ratent No. 1,688,292 to A. Weaver. I have found that by suitable circuit arrangements, it isl possible to utilize the otherwise undesirable inherent electrostatic capacity of a photo-electric cell as the controlling factor in a translating circuit.v

In other words, a system such as disclosed in the said Weaver patent treats the inherent capacity as a disadvantage which is to beovercome or balanced out. while I have found that it is'possible tol utilize that capacity in a practical way.

AccordinglinaI feature of the invention relates to a novel light signal translating arrangement utilizing the inherent electrostatic capacity variations of a photo-electric cell in response to light signals.

Another feature relates to a photo-electric cell translating arrangement wherein no D. C. polarizing potentials are necessary in operating the cell, and a carrier voltage is applied to the "50 cell and limited in amplitude so that the cell operates substantially entirely as a variable capacitor as distinguished from a variable 'resistor. I

photo-electric cell construction.

A still further feature relates to the novel organization, arrangement and relative location and interconnection of parts which constitute an improved light translating system.

Other features and advantages not specifically enumerated will be apparent after a consideration of the -following vdetailed descriptions and the appended claims.

1 Referring-to the,drawings,

Fig. 1 is a schematic wiring diagram of a typical vmodulating system embodying features oi the invention. i

Fig. 2 is a modification of Fig. 1 employing two photo-electric cells.

Fig. 3 is another modication of Fig. i employing a duplex photo tube.

Fig. 4 is an ele'vationalview of a duplex photoelectric tubev according to the invention.

` Fig. 5 is another View oi Fig. 4 with part of the bulb broken away to show the interior construction.

Fig. 6 is a bottom plan View of Fig. 4.

Fig. 7 is a sectional view through the bulb of Fig. 4 taken along theV line 1-1 toshow a topplan view oi the electrode mount.

In accordance with the present invention, the

voltages applied to the photo-electric cell or cells V are purposely limited so that `they take no part in rendering the cells conductive to D. C. In other words, none of the usual polarizing sources are required.- and in fact I have found that even in to a cathode oi a photo-electric cell. is in.

.A further feature relates to a novel iorm oi 55 tended to mean the electrode upon which the light signals are projected and which forms.l

clouds of electrons in response to such signals.

` I have found that present day photo-electric cells, especiaHy those of the high vacuum type, can be operated as substantially pure variable capacitors, as distinguished from resistors or conductance controls. However, in order to so operate s'uch cells, I have found it necessary not only -erably inv tlie form ot 'adjustable Weaver patent, the D. C. polarizing batteriesl employed on the cell cause the cell to be conductive and to act as a variable resistor when the carrier is impressed thereon. Consequently, when the cell is subjected to light variations,

complex modulation products arise in the cell circuit. Furthermore, since the cell acts as a variable resistor, it is necessary to provide some means of balancing out the normal resistance of the cell, as well as means for balancing out its capacity with respect to ground. Furthermore, in such priorv arrangements, a D. C. conductive path must be provided acrossl the electrodes of the cell and for this reason relatively high polarizing voltages. as high of gas cells are required. I have -found that this D. C. path reduces the sensitivity of Athe cell and increases the diillculty of balancing the network into which the cell is connected. In accordance with the present invention, the cathode and anode of the photo-electric cell are substantially conductively isolated from each other, so far as D. C. is concerned, during the entire range of light signals to which the cell is` exposed. While in its preferred form, the invention provides no D. C. circuit in series with the electrodes of the cell, nevertheless to a certain extent the cell can be operated with very small D. C. voltages applied thereacross so long as these voltages are not ot suilicient magnitude to cause the cell to act as an equivalent variable resistor. Thus, I have found that voltages of about or 6 volts may-be impressed on the cell without appreciably introducing any variable resistor characteristics and yet the cell operates as a substantially pure capacitor.

Referring to Fig. -1, there is shown schematic form. a light sensitive cell I. of an wellknown construction having the light sensitive electrode or cathode 2 which emits clouds of electrons when light impinges thereon; and a cooperating spaced anode 3 which is not light sensitive. The electrodes 2 and 3 are enclosed in a. suitable'highly evacuated glass bulb having a suitable light, shield for restricting the entrance of light to the cathode 2 from the signalling light source. A source 4' of carrier current prefa substantially 'sinusoidal audio frequency alternating current is connected across the electrodes 2 and 3 in vseries with an condenser 5. In .accordance with the invention, the source 4 is adjusted so that itsmaximum voltage swing is well below the volt-k age necessary to render the space between electrodes 2 and 3 conductive as a D. C. path. Thus the source 4 may have its amplitude `limited to a maximum of approximately two volts, which is lnsumcient to draw any substantial quantity of photo-electrons from electrode 2 to electrode -3 when the latter is energized by the positive halfwaves.

Connected across source 4 is a potentiometer resistance 6, the mld-point 1 o! which is grounded of the cell and it is pos' as90voltsinthecase.

. of apparent capacity in cell I,

or connected to a suitable steady base potential which may be positive or negative with respect to ground. The conductor l connecting electrode 3 and condenser B is also conn( ed to the control grid 9 oi a suitable amplifier tube In whose cathode II is connected to the same base potential as that of the point 1. The control grid 0 is biased with respect to cathode II by the series resistor I2 and battery I3 which has its positive terminal connected to the cathode II. Preferably, the battery I3 is of about I to 5 volts and is not materially higher in voltage than the maximum swing of the voltage source 4. I have found that when the voltage of source 4 is limited as above described, variations of light impinging on cathode 2 cause the cell I rto act as a substantially pure variable capacitor. Consequently, if the point 1 is the electrical mid-point of resistor 3,and if capacity i is adjusted so that it is substantially the same as the capacity between electrodes 2 and 3 with the cell I dark. then the carrier vol e across points 1 and I4 is approximately zero. Consequently, there will be approximately zero carrier voltage impressed on grid 9. It will be understood of course that the point 1 may be adjustable along the resistor 3 to balance the network. When light shines on cathode 2, the apparent capacity between electrodes 2 and 3 increases, and unbalance of the bridge circuit occurs, resulting in a carrier voltage across points 1 and I4 and a corresponding voltage difference between grid l and cathode II. This impressed voltage will therefore have the frequency of the source 4 but will have its amplitude modulated in accordance with the light signals impinging on cell I. 'I'he modulated carrier signal is then amplified in the amplifier tube t and in one or more succeeding amplifier stages I8. While I am not prepared to give the exact reason why the light variations result in a change one probable explanation is that since 'the D. C. voltage applied acrosselectrodes 2 and 3 is purposely limited so that it is below that sumcient to draw any appreciable quantity of photo-electrons emitted from cathode 2 to anode l, and since the applied carrier voltage from source 4 is likewise limited, the light impinging on cathode 2 results in photo-electron clouds or sheaths of varying thickness or depth adjacent cathode 2. Thus the cell I acts like a substantially pure variable capacity and can be readily balanced against the capacitor B. I have found that the use of any substantial D. C. voltage across the electrodes of cell I such as the usual D. C. polarizing voltages which are ordinarily of the order of 20 to 90 volts, reduces the sensitivity of the cell when reliance is placed on its capacity variations for modulation purposes. 1f the ordinary polarizing potentials are applied across the cell and if care is not exercised to limit the voltage swing of the source 4, the cell would act mainly as a variable resistor in response to light variations, and the modulating components will then generally exceed the intensity o! the applied carrier waves making it necessary to nlter out, cr balance out,

such modulating components in order to prevent exaggerated outlines and secondary or stutter outlines where the system is used Afor picture transmission or similar purposes.

If in the arrangement of Fig. 1 a slight low frequency modulating component should appear on grid 3, this low frequency component can be reduced to a negligible factor by proper design of the coupling between ampliiler stages I and I3. One method is to design the coupling transfarmers I6, I 1, so that they are very ineillcient at the lower modulating frequencies but have high eilciency at the carrier frequency of source 4, for example 1800 C. P. S.

The output of amplier I can be applied to any suitable 'transmission channel whether oi' the wire or radio type, and if cell I is illuminated under control of any well-known type or iacsimile scanner, the picture elements are translated into a modulated audio frequency carrier which can be received and translated back into a facsimile reproduction by any well-known facsimile receiving machine.

Referring to Fig. 2, there is shown a modification wherein the main or modulating cell' I3 which corresponds to cell I (Fig. l) is balanced by a second similar cell I3 whose anode 2u is connected directly to anode 2| and whose cathode 22 is` connected through potentiometer resistance 23 to cathode 24. If desired, a small amount of adjustable light from the same source which illuminates cell I8, or from a separate adjustable source, may shine on cathode 22 to adjust the capacity of cell I8 for balancing purposes. In Fig'. 2 as in Fig. l, no D. C. polarizing source is connected in the circuit of cells I 8 and I3 and the applied carrier from source 25 is llmlted in amplitude so that the light signals impressed on cells I3 and I3 cause them to act as substantially pure capacitors which can be balanced readily against each other. The arrangement of Fig. 2 has the advantage over that of Fig. l in that if for any reason there is an unavoidable resistance leakage between electrodes 2I and 24, this can be readily balanced out by the corresponding resistance leakage of cell I9. Such resistance leakage cannot be balanced out with a. device such as the condenser 5 (Fig. 1). In the embodiment of Fig. 2, the amplication of the modulated carrier is eifected by' pentode tube ampliers 26, 21, which are coupled together by resistance-condenser coupling as shown. The coupling capacitor 28 is preferably chosen of such a low value that any undesired low irequency modulating components arising in the bridge are greatly attenuated while the carrier frequency of source 25 is amplied emciently. inasmuch `as the connections for pentode tube amplifiers are well-known in the art, further description thereof is deemed unnecessary beyond stating that the rectangles 2l and 30, represent common or separate sources of high D. C. potential supply for the various electrodes of the pentodes.

Whereas in the embodiment of Fig. 2, the cells I8. I8, are shown as separate tubes, I have found, when using the ,system of Fig. 2, it is more eilicient and practical to use two photo-electric cells contained inthe same tube or envelope. A preferred embodiment of such a duplex cell is shown in Figs. 4 to 7, wherein the interior electrode arrangement may be the same as that of an RCA photocell type 920. "I'he glass envelope or bulb'3l is highly evacuated and ,has sealed therein the usual reentrant stem or press 32 of the cathodes is in the form of a cylindrical sector symmetrically spaced with relation to the central vertical axis of the tube mount. The rods 33 and 38 adjacent the press SZ-are'bent at right angles and are then extendedrvertically up. wards and parallel to each other to constitute anodes 44, 45, located approximately along the axes of curvature of the associated cathodes 42, 43. In order to maintain the spaced relation at the upper ends of the electrodes. wires 43, 4'I, pand 49, extending from the respective electrodes are rigidly united and insulated from each other by a suitable glass bead 50. The wall of the bulb is enclosed in a light-proof housing or Y sheath 5I which may take the form oi a coating of black wax, tar or the like, except that a portion of the bulb is left uncoated to form a window 52 through which the light beam enters. As will be seen from Fig. 7, the window extends from the point 53 to the point 54 .or approximately one-fourth of the periphery of the bulb. The window is so arranged with respect to the electrode systems so that when the 'light is pro- Jected in the direction of the dotted arrow (Fig. 7), the entire width oi' cathode 42 is illuminated but only a small fraction of the light leaks by reflection or otherwise on to the cathode 43. In order to adjust more precisely the electrostatic balance between the two electrode systems of the duplex cell, there is provided a small heavy bridging wire or metal strap 55 which is soldered across the prongs 3l and 39 connected respectively with anodes 44. 45.. Fastened to strap 55 is an L-shaped metal member which has a portion il (Fig. 5) extending parallel to the cathode prongs 4I), 4I. 'Ihe member 56 is of suiilciently heavy metal stock so that by bending the leg B3 through which are sealed the 'various lead-in and support wires 33, 34, I3, 3i. I'he bulb is provided with' any well-known form of shell base 31 having the usual contact prongs 38, 39, 40, 4I. Attached to the lead-in wires 34, 35 as by Welding, are the light sensitive cathodes 42, 43, which are of metal having their concave surfaces proceased or coated to render them electrically emissive when light shines thereon. Preferably. each the leg 5l can be varied in position with respect to prongs 4I), 4I,thus varying the relative capacitance between. the anodes and cathodes of the electrode systems.

In order to increase further the sensitivity of the duplex cell, a conductivefmember 58 can be fastened to the exterior of bulb 3| and electrif cally connected by wire 59l to the strap I5. As shown more clearly in Figs. 4 and 7.1the member 53 is in the form of a ne wire in the shape of a rectangular loop which is fastened to the bulb by the light impervious wax coating above de scribed. Preferably, although not necessarily, the member 58 has its horizontal sid adjacent the edges of the vvindow 52 so as not to interfere with the light through the window. Preferably also, the member 58 extends from the point 53 (Fig. 7) to the point 40 so that it electrostatically overlaps both electrode systems.

Referring to Fig. 3, there is shown a preferred form of circuit arrangement embodying the duplex cell of Figs. 4 to 7. The parts of the cell corresponding to those of Figs. 4 to 7 bear the same designation numerals. The source of audio frequency carrier current BI which may be for example 1800 C. P. S., is connected to the primary 82 of a coupling transformer which has -two secondary windings' 43, 64. The electrical mid-points of the windings i3, 64 are connected together by the conductor 35. The ends of wind-V ings 33 are connected to contact prongs 4l, 4I, which in turn are connected to the respective` cathodes 42, 43. The anodes 44, 4I are connected together by the strap 55, and this strap is connected to the amustable member se, which can be adjusted with respect to the prongs 40, 4I as indicated by the curved arrow and as described above in connection with Figs. 4 to 7. The supby the dotted line 3i.

signal or v made without departing ot the invention.

plementary matically by envelope 3l is likewise metal member Sl is indicated schethe dotted line $8 in Fig. 3, and the indicated schematically The secondary winding 6l is` connected to ground through the potentiometer resistance 8 5 and likewise the adjustable member i8 is connected to ground through the resistance 61. The

purpose of the auxiliary transformer winding 64 is to enable a small amount ofV carrier signal to beappliedy to the amplifier tube C8 to represent the "shadow" signal. Thus, the shadow signal may beimpressed on the amplifier without disturbing the electrostatic capacity balance between the two sections of .the duplex cell 3i or without appreciably changing the applied voltage -to the cells. The winding il has a very small number of turns compared with the winding I3 and the phase and amplitude oi the shadow signal applied to tube I8 can be controlled by the potentiometer 88. Ifthe contact arm BI of this potentiometer is on one side oi the center of resistance 86, positive modulation is obtained in the carrier for the complete range of shade values to be transmitted. On the other hand, if the arm Bl is on the other side of the center of resistance 86, negative modulation of the carrleris obtained ior the range of shade values to be transmitted. Thus, the displacement oi arm 89 from the center oi resistance l0 controls the amount of initial shadow signal corresponding to positive modulation or it controls the amount of initial signal or high light'signal corresponding to negative modulation. tode amplifier tube which feeds another similar pentode amplier tube 6! through the usual resistance-condenser coupling.A The coupling condenser 10 is made very small in order to attenuate greatly any undesired modulating components in the carrier but the amplliler as a whole is designed to pass and amplify efliciently the carrier frequency from source il.

Normally, the duplex cell is balanced correspending to "shadow" signals or corresponding to Ahigh-light signals. Ordinarily, since the duplex cell is provided with identical electrode systems, the interelectrode capacities between the respective cathodes and anodes will be substantially the same. However, ii one cell happens to be slightly diner-ent from the other, it can be balanced by allowing an appropriate amount of leakage light to shine on the cathode Il. However, if this method of balancing is notdesired, the member I6 may be adjusted towards or away from the prong Il to vary the relative capacities of the two sections'oi the cell for balancing. It will be understood of course that in achieving this initlal balance, the source 6| may be disconnected and thc member it moved to adjust the relative capacities between the two cells, the balance or predetermined ratio of the capacities is being determined by a suitable indicator device (not shown) and the output of the amplifier 68 or the amplier 89.

Various changes and modificationsY may be This application is a' continuation-impart of application Serial No. 273,162, filed May 12, 1939,

Patent No. 2,212,808, August 27, 1940. l What I claim is: 1. A modulating system comprising a Wheat.

stone bridge, a variable arm of which is constituted of a photo-electric cell responsive tolight signals, a source of a carrier voltage connected The tube 8l may be a pento said bridge, said cell being free from any D. C. polarizing potentials of suillcient magnitude to draw a substantial quantity of photo-electrons from the cathode to the anode, an connected to said bridge, and means said tube a small amount of unmodulated carrier voltage independently of the adjustment of said bridge.

2. A modulating system comprising a pair of similar photonelectric cells, a source of carrier voltage. a Wheatstone bridge circuit arrangement interconnecting said cells, said cells being substantially free from D. C. polarizing potentials, means to adjust the relative inherent electrostatic capacitances of said cells to balance the bridge, an amplifier tube connected to the bridge and controlled by the unbalance thereof, and means to apply to said tube a small quantity of unmodulated carrier without disturbing the balance of said bridge. f

3. A modulating system comprising a duplex photo-electric cell, a source of carrier voltage, a transformer connected to said source and having its secondary winding connected with said duplex cell to iorm a Wheatstone bridge, means to adjust the inherent electrostatic capacitance of the sections of said cell to balance the bridge, an amplifier tube connected to said bridge to be controlled by the unbalance current thereof, and another secondary winding for said transformer connected to said tube to apply a predetermined small amount of unmodulated carrier without disturbing the balance of said bridge.

4. A modulating system according to claim 3 in which means is provided to adjust the relative phase and amplitude of said unmodulated carrier with respect to the unbalance current of the bridge.

5. A modulating system comprising a pair of transformer windings each energized from a common source of audio frequency carrier volt age. means interconnecting the electrical midpoints of said windings, a ilrst photo-electric cathode and its cooperating anode, a second photo-electric cathode and its cooperating anode, means interconnecting said cathodes and anodel with one of said transformer windings to form a Wheatstone bridge, an ampliiler tube connected v to said bridge and controlled by the unbalance from the spirit and scope current thereof, and a potentiometer connected across the other ot said transformer windings the variable arm of said potentiometer being connected to said tube to apply a predetermined amount of unmodulated carrier to the tube without disturbing the adjustment of 4said bridge.

6. 'The method-of modulating a carrier voltage in accordance with light signals and employing only the photo-capacitance variations of a photoelectric cell-which includes the steps of applying the'carrier voltage to the cell, balancing out the inherent electrostatiecapacity of the cell to represent a given lightA condition, limiting the maximum amplitude ot said carrier voltage as applied to said cell to a value where omy the photo-capacitance of the cell varies whilev substantially eliminating any uni-lateral conductivampliiier tube to apply to A ing out the inherent electrostatic capacity ofthe cell to represent a given light condition, and limiting ythe maximum amplitude of the carrier voltage applied to the cell so that said cell symmetrically responds to the positive and negative half waves. of the carrier when the cell is illuminated by light signals and is substantiallyy entirely free from asymmetric conductivity normally tending to result from the positive half 10 waves of the carrier above said amplitude.

AUSTIN G. COOLEY.

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